### What changes were proposed in this pull request?
`/api/v1/workers/events` should support `None` `eventType` to align `/sendWorkerEvent`.
### Why are the changes needed?
Legal event types of `/sendWorkerEvent` are `None`, `Immediately`, `Decommission`, `DecommissionThenIdle`, `Graceful`, `Recommission`. But `/api/v1/workers/events` does not support `eventType` with `None` type.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
`ApiV1MasterResourceSuite#worker resource`
Closes#2732 from SteNicholas/CELEBORN-1477.
Authored-by: SteNicholas <programgeek@163.com>
Signed-off-by: Shuang <lvshuang.xjs@alibaba-inc.com>
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